There is an array whose elements are same items in random order. I want to reorder the array such that items with high frequencies appear first and then the items with the lowest frequencies appear last.
Basically, what would be the code to return the result below?
const a = [1,5,3,3,2,3,4,3,3,2,4,4,4];
const result = [3,3,3,3,3,4,4,4,4,2,2,1,5];
Thanks a lot in advance!
Calculate the frequency and then sort the array based on the frequency:
let a = [1,5,3,3,2,3,4,3,3,2,4,4,4];
const freq = a.reduce((c, v) => (c[v] = (c[v] || 0) + 1, c), {});
a.sort((x, y) => freq[y]-freq[x] || y-x);
console.log(a);
frequency using reduce: const frequency = a.reduce((c, v) => (c[v] = (c[v] || 0) + 1, c), {});You need to collect same values, sort by length and get a flat array.
const
array = [1,5,3,3,2,3,4,3,3,2,4,4,4],
result = array
.reduce((r, v) => {
const temp = r.find(([w]) => v === w);
if (temp) temp.push(v);
else r.push([v]);
return r;
}, [])
.sort((a, b) => b.length - a.length)
.flat();
console.log(...result);
sort((a, b) => b.length-a.length || b-a) or sort((a, b) => b.length-a.length || a-b) might be better?You can collect a frequency map, sort it, and generate resulting array by value and frequency.
const a = [1, 2, 3, 4, 3, 3, 2, 4, 4, 4, 5];
const result = [...a.reduce((freqs, item) => freqs.set(item, (freqs.get(item) || 0) + 1), new Map).entries()]
.sort(([, a], [, b]) => b - a)
.flatMap(([value, length]) => Array.from({length}).fill(value))
console.log(JSON.stringify(result))
function compareValues(a, b) {
return (a.localeCompare && b.localeCompare)
? a.localeCompare(b)
: ((a < b && -1) || (a > b && 1) || 0);
}
function compareValueLists(a, b) {
return (b.length - a.length) || compareValues(a[0], b[0]);
}
function collectEqualValue(collector, value) {
const { index, list } = collector;
let valueList = index[value];
if (!valueList) {
valueList = index[value] = [];
list.push(valueList);
}
valueList.push(value);
return collector;
}
const sampleList = [5, 1, 3, 3, 2, 3, 4, 3, 3, 2, 4, 4, 4];
console.log(
...sampleList
.reduce(collectEqualValue, { index: {}, list: [] }).list
.sort(compareValueLists)
.flat()
);You can also use an object to store how many times each number appears and sort the array on that basis.
let a = [1, 2, 3, 3, 2, 3, 4, 3, 3, 2, 4, 4, 4];
let obj = {};
for (num of a) {
if (obj[num] === undefined) {
obj[num] = 1;
} else {
obj[num] += 1;
}
}
Object.keys(obj)
.sort((a, b) => obj[b] - obj[a])
.map((key) => Array.from({ length: obj[key] }, () => parseInt(key)))
.flat();
/*
OUTPUT: [3, 3, 3, 3, 3, 4, 4, 4, 4, 2, 2, 2, 1 ]
*/
